By accident, my apartment has this strange natural air-conditioning. I noticed it when I first moved here. Every summer when it gets hot, I just have to open the front and back windows, and an incredible draft blows through. I'm feeling it right now, and the blow rate is roughly equivalent to the medium setting on my 45 watt fan.
It blows constantly, always in the same direction, and I wondered about it for a while. The explanation seems to be this: Out back there's a black asphalt parking lot, which is usually about half full. I'm figuring that the air rises off the asphalt, creating a convection current which draws in cool air from the shaded courtyard at the front of the building.